The preparation of nuclear substituted chloro-compounds by reaction of chlorine with an alkylbenzene is well known. Thus, for example, the reaction of toluene with chlorine to form monochlorotoluene is well known and of considerable commercial importance. Such reactions are commonly carried out in the presence of a chlorination catalyst such as antimony chloride, ferric chloride, ferrous sulfide, aluminum chloride or the like in combination with sulfur monochloride. The usual products of such reactions are a mixture of various monochlorinated and/or polychlorinated compounds and various position isomers of these. For example, in the liquid phase substitution-chlorination of toluene, by reaction of chlorine and toluene, to form monochlorotoluene, the usual product is a mixture of orthochlorotoluene and parachlorotoluene which may, in addition, contain varying amounts of other chlorinated products such as dichlorotoluene, polychlorotoluenes and benzyl chlorides. Of the major reaction products, that is orthochlorotoluene and parachlorotoluene, the latter is the most commercially valuable. Based on the availability of two ortho- positions and one para-position, as chlorination sites a monochlorotoluene product having a ratio of orthochloro:parachloro isomer of about 2:1 would be predicted. In the past, considerable effort has been expended in attempts to direct the chlorination reaction in such a manner as to lower the ratio of orthochlorotoluene to parachlorotoluene, that is, to discover reaction conditions under which the formation of parachlorotoluene is favored.
In co-pending application Ser. No. 601,219 (Case 3387), filed Aug. 1, 1975, the disclosure of which is hereby incorporated by reference, it is disclosed that a product having a substantially lower ratio of orthochloro:parachloro isomer is obtained when an alkylbenzene is reacted with chlorine in the presence of a catalyst system comprising a Lewis acid an a thianthrene compound.
On an industrial scale, chlorination reactions such as those discussed hereinabove are commonly carried out using equipment which is fabricated, at least in part, from iron or iron containing alloys. It has been found that under such conditions, utilizing a catalyst system comprising a Lewis acid and a thianthrene compound, the desired lowering of the ortho-para isomer ratio, although substantial, is considerably poorer than that achieved when non-iron-containing equipment is employed. It is believed that iron and/or steel equipment, dissolves slightly under the corrosive conditions present in chlorination reaction mixtures, to increase the concentration of iron to significantly higher levels than are present when the chlorination reactions are conducted in non-iron-containing equipment. Although the exact mechanism is not known, it is believed that the presence of excessive amounts of dissolved iron in the chlorination reaction mixture decreases the ability of such catalyst systems to specifically catalyze para-chlorination.
Accordingly, it is an object of this invention to provide an improved process for the para-directed chlorination of alkylbenzenes. It is a further object to provide a simple and effective method whereby the para-directed chlorination of alkylbenzenes, in the presence of a catalyst system comprising a Lewis acid and a thianthrene compound may be carried out in the presence of, or in contact with, iron-containing equipment or materials, without substantial deleterious effect on the enhanced para-directional activity of such catalyst system. It is a still further object of the present invention to provide an improved process for the commercial production of parachlorotoluene.